1) Field of the Invention
The present application relates to a high-pressure process for preparing pure melamine by pyrolysis of urea in a vertical synthesis reactor, and to a reactor for carrying out this process.
2) Description of the Related Art
In the high-pressure processes for preparing melamine, generally urea melt and optionally gaseous ammonia are reacted in the absence of a catalyst, for instance at temperatures between 325 and 450° C. and pressures between 50 and 250 bar, to give liquid melamine and offgas consisting mainly of ammonia and carbon dioxide. The liquid melamine which, in addition to unconverted urea, also comprises by-products is subsequently worked up, for example by quenching with water, by sublimation or by decompression under certain conditions, and the pure melamine is subsequently isolated.
The melamine reactors known from the conventional melamine processes are typically vertical tank reactors of the loop reactor type, as described, for example, in AT 409 489 B. Such a reactor has an inner central tube and a heating bundle with circulating salt melt between central tube and reactor wall. The salt melt serves to provide the heat needed for the endothermic melamine synthesis. The urea melt and optionally the ammonia are fed in the lower region of the melamine reactor and react in the outer space between the salt melt-conducting bundle tubes to give melamine melt and offgas. Owing to its low density, the reaction mixture rises upwards, where a separation between melamine melt and offgas takes place. While the offgas is discharged from the reactor and fed to an offgas scrubber, the melamine melt flows downwards in the central tube by virtue of gravity, encounters freshly introduced urea melt there and rises again upwards in the reaction space between the bundle tubes. This circulation of the melamine melt in the synthesis reactor is referred to natural circulation and ensures a certain residence time in the reactor, which is intended to serve for maximum urea conversion to melamine. After the residence time, the melamine melt is discharged via an overflow in the upper reactor region and sent to further workup.
A disadvantage of this melamine reactor is the fact that that ratio of the heat exchange surface area of the bundle tubes to the reaction volume is relatively small, so that relatively high salt melt temperatures are needed for the supply of the heat of reaction needed. These cause increased corrosion on the tube bundle, so that it is necessary to chemically clean the tube bundle annually, which is undesired owing to the production shutdown.
A further disadvantage results from the attainment of a wide residence time distribution in the one-stage loop reactor, i.e. the proportion of unconverted urea in the discharged melamine melt is comparatively high. Since the unconverted urea is discharged together with the by-products in the subsequent melamine workup, this equates to a melamine loss.
EP 0 612 560 describes a vertical melamine reactor consisting of three sections, in which the melamine synthesis takes place in the lowermost sector. The urea melt is conducted downwards via a downpipe from the uppermost region, exits there and reacts in the presence of ammonia between salt melt-conducting tubes to give melamine melt and offgas. The internal melamine circulation in the reactor interior takes place via a central tube. The melamine melt passes through a diaphragm into the sector above, where the offgases are removed from the melt. While the melamine melt is discharged and sent to further workup, the offgas is fed to the uppermost sector, an offgas scrubber, where it is cooled and subsequently discharged.
This reactor too has the disadvantages mentioned of high corrosion and unconverted urea, since it equates to a one-stage loop reactor with regard to reaction control.
WO 99/00374 describes a multistage melamine reactor consisting of a plurality of apparatuses connected in series. The synthesis reactor is a conventional loop reactor. The melamine melt separated from the offgas is subsequently fed to a horizontal tubular reactor in which the urea conversion is to be completed. The reaction mixture is subsequently introduced into an offgas separator and the resulting melamine is sent to further processing. In one variation of the process, a CO2 stripper is connected downstream of the first tubular reactor, then the pressure of the melamine melt is increased, before a further tubular reactor stage and finally an offgas separator follow.
The reactor described has the disadvantage that numerous apparatuses connected in series are needed, which causes high capital costs and a complex design of the plant. In addition, the corrosion problem and the incomplete urea conversion in the loop reactor section exist here too. Since the entire reactor space is taken up by liquid phase in the tubular reactors described and the offgases formed in the melamine formation cannot be removed continuously, it is not possible in these reaction tubes to achieve full urea conversion.
AT 410 210 B discloses a process for preparing melamine by pyrolysis of urea, which urea is introduced into a tank reactor and melamine melt formed in the tank reactor is cooled in a downstream cooling reactor by means of supply of a small amount of urea. The cooling reactor used may be any desired reactor, for example a stirred reactor, a falling-film reactor or else a combination reactor whose upper section is in the form of a tank reactor and whose lower section is in the form of a falling-film reactor. The amount of urea added to the cooling reactor is from about 1 to 5% by weight of the total amount of urea needed to prepare the melamine. In this process, the same disadvantages with regard to the melamine synthesis occur as in a one-stage loop reactor.